1. Field of the Invention
The invention relates to transmission line devices for absorbing microwave energy.
2. Description of the Prior Art
In transmission systems for propagation of microwave energy, the problems of termination of such systems raises problems with respect to dissipation of heat with high average and peak pulse power levels of energy. Impedance matching, bandwidth and voltage standing wave ratio (VSWR) are important factors to be considered in providing for substantially wave-reflectionless characteristics with absorbing devices. In addition a microwave dissipative load is frequently required in the art for measurement of high average power levels utilizing well known calorimetric techniques.
Loads of the type disclosed in U.S. Pat. No. 3,044,027, issued July 10, 1962 to D. D. Chin et al, provide for the circulation of a liquid which becomes heated upon the impingement of the microwave energy and the rise in temperature is calibrated to provide a corresponding reading indicative of the power level. Another example of prior art teaching is found in U.S. Pat. No. 3,597,708, issued Aug. 3, 1971 to Henry W. Perreault, and assigned to the assignee of the present invention. In this embodiment a coolant is circulated through concentrically disposed conductive members to define a coaxial reentrant folded-line path whereby the overall length of the load is substantially reduced. Other embodiments of prior art teachings include energy absorption means, such as silicon carbide provided in a wedge form, having a surrounding cooling jacket for removal of the generated heat. Other suggested embodiments in the prior art include the provision of a quarter-wave window block of a dielectric material together with means for directing a stream of a dielectric liquid over the face of the block for absorbing the microwave energy absorbed from the source.
All of the prior art embodiments are substantially costly in implementation and some have cumbersome overall lengths. The problem of providing a suitable dissipative load becomes increasingly important in the handling of high powers in very high frequencies with very short wavelengths, for example, the eight millimeter band with frequencies in the 30 thousand MHz range where the waveguide is exceedingly small and conventional load techniques cannot be implemented. In addition to the power absorption characteristics, a load must provide for impedance matching to the transmission line which is reasonably independent of temperature, as well as being relatively insensitive to surrounding environmental conditions. Voltage standing wave ratio (VSWR) ratings of the load terminations should also be less than 1.2 in order to be acceptable. A need arises, therefore, for the provision of new and novel dissipative load structures having high average and peak power handling capabilities over a reasonably broad frequency band for use in the very, ultra and super high frequency portions of the electromagnetic energy spectrum.